Fungus gnats
Small black gnats of 2–4 mm run over the compost and flutter around the pot. Their glassy larvae with black heads live in the top layer of compost and nibble fine roots, especially of cuttings and seedlings.
Fungus gnats lay their eggs in compost that is constantly moist and rich in organic matter.
What can you do?
- Let the top 2–3 cm of compost dry out well before watering again, or water from below via the saucer.
- Push yellow sticky traps into the pot to catch the adult gnats.
- Cover the compost with a layer of sand or fine grit so they can no longer lay eggs in it.
- For a stubborn infestation, water in nematodes (Steinernema feltiae), which kill the larvae.
Prevention
Don’t overwater, especially in winter, and keep bags of compost closed. Use fresh, clean seed compost for sowing.